Norwalk Virus

1. Norwalk Virus Phuong D. Nguyen

2. Introduction • Family Caliciviridae • Small, nonenveloped RNA viruses • Distinctive • Cup-shaped depressions on a spherical capsid surface • Size (27 to 35 nm) • Genome encodes a single major polypeptide • Cause of mucocutaneous and respiratory tract lesions in animals such as swine and cats • Norwalk virus is the best known of the human caliciviruses • Humans are the only known hosts • Has not been cultivated in vitro

3. Structure • Small, round, 27 nm diameter, nonenveloped • Amorphous surface • (+) ssRNA, 7.5 kb • Single structural protein, 60 kD

4. Acute viral gastroenteritis Infectious dose Unknown Presumed to be low Signs and symptoms Abdominal pain Vomiting: non-bloody Diarrhea: watery, non-bloody Nausea Headache Low-grade fever Malaise Myalgia Incubation period 24 to 48 hours Cases can occur within 12 hours of exposure Complications Dehydration among young and elderly Medical attention Recovery 24-60 hours Complete No long-term sequelae Clinical attributes

5. Classification • Norwalk and Norwalk-like viruses belong to the genus of Noroviruses and are named after location of outbreak • Norwalk, Ohio • Snow Mountain, Colorado • Taunton and Southampton, England • Otofuke and Sapporo, Japan • Sapovirus • Formerly known as Sapporo-like virus • Genetic homology yet distinct antigenicity among Norwalk and Norwalk-like viruses by immune electron microscopy studies

6. History • In 1968, 50% of 232 students and teachers at a Norwalk elementary school developed acute gastroenteritis • In 1972, the Norwalk virus was discovered by Dr. Al Kapikan from the National Institute of Allergy and Infectious Disease by immune EM of stool samples from the 1968 outbreak

7. Multiplication • Current studies on the replication strategy of Norwalk virus • Cloning and sequencing of genome • Three open reading frames comprise a single, positive strand of RNA • A single capsid protein is encoded by one of the reading frames

8. Pathogenesis • Fecal-oral route • Eating contaminated food • Raw shellfish • Salad ingredients • Drinking contaminated water • Most common source • Municipal to recreational • Person to person • Secondary transmission • Direct contact

9. After ingestion, infection of the mucosa of the proximal small intestine, damage of microvilli, and malabsorption of D-xylose, lactose, and fat are caused by the virus • No invasion of the colon and thus, no fecal leukocytes in stool

10. Host defenses • Most adults have serum antibodies • No protection against infection • Markers or risk factors • Studies have shown short-term immunity persisting for about 12 weeks • Strain-specific • Repeated infections likely • Further studies • Genetic susceptibility • High risk of severe infection among those with O blood type • Repetitive past exposures

11. Epidemiology • 50% of all outbreaks involving acute infectious, nonbacterial gastroenteritis in the U.S. caused by the Norwalk virus • Second most common cause of illness in U.S. after influenza • Occurrence typically in older children and adults • Serum antibodies acquired during early adulthood • Camps, schools, nursing homes, military populations, cruise ships • Infection is worldwide based on seroprevalence studies • Some evidence that Norwalk is spread by the respiratory route • Transmission through aerosolized virus-containing vomitus • Very rapid secondary spread of infection

12. Diagnosis • Virus identified in stool specimens • Antibodies measured in serum samples by immune EM or immunoassays • Few research laboratories with necessary reagents from human volunteer studies • Limited diagnosis to community outbreak settings • 27 state public health laboratories can test for virus in stool or contaminated food and water by reverse transcriptase polymerase chain reaction (RT-PCR)

13. Prevention • Frequent hand-washing with soap and warm water • After toilet visits • After changing diapers • Before eating or preparing food • Cook all shellfish thoroughly and wash fruits and vegetables • Clean and disinfect contaminated surfaces • Sanitary disposal of sewage • No food preparation by persons infected during symptoms and 3 days after

14. Control • No specific antiviral therapy for Norwalk or Norwalk-like viruses • Long-lasting immunity by development of a vaccine not likely

15. Recent outbreaks • September 2, 2003: 340 sick passengers and crew on a cruise ship Regal Princess docked in Manhattan • July 7, 2003: 28 US Airways passengers coming back to North Carolina from an Arizona Indian reservation were ill before they boarded the plane • December 4, 2002: 60 ill passengers aboard the Disney cruise ship Magic in Florida • November 28, 2002: ER at Toronto’s Sunnybrook Hospital closed • March 13, 2002: 50% of 550 guests infected at an Oscar party

16. CDC’s vessel sanitation program • 140 ships participate and given scores on a 100-point scale • A clean bill of health given to a score of 86 and over • 8 ships with a 100 point score • Norwegian Star • Carnival Legend • Brilliance of the Seas • Aurora • Star Princess • Seven Seas Navigator • The World • Wind Surf